Structure of Insulin: A and B Chains and Disulphide Bonds
Biology · Biotechnology and Its Applications · NEET
Mature insulin is a small protein made of TWO short polypeptide chains, called chain A and chain B, which are held together by covalent disulphide (S-S) bridges, NOT by hydrogen bonds. Memory hook: "A and B, held by S-S" - two chains, disulphide bridges. NEET asks this almost every year, so remember: disulphide bridges (not hydrogen bonds) is the linkage.
Mature insulin has two short chains, A and B, joined by covalent disulphide (S-S) bridges. Remember: disulphide bridges, not hydrogen bonds, and no C-peptide in the mature form.
Your doubts, answered
Are the insulin chains linked by hydrogen bonds or disulphide bonds?
By disulphide bridges (S-S bonds), which are covalent and strong. This is the single most tested point. NEET repeatedly puts 'hydrogen bonds' as a wrong option to trap you. The A and B chains are joined by disulphide bridges between cysteine amino acids, not by weak hydrogen bonds.
How many chains does mature insulin have?
TWO chains only: chain A and chain B. A common trap says 'three chains - A, B and C'. That is wrong. The C-peptide is present only in proinsulin (the immature form) and is removed during maturation, so mature insulin has just chains A and B.
What exactly are the A and B chains?
They are two short polypeptide chains (short strings of amino acids) that make up one insulin molecule. Both together form the functional hormone. Insulin is a peptide hormone, so both chains are made of amino acids joined by peptide bonds within each chain, and the two chains are joined to each other by disulphide bridges.
Is insulin a protein or a carbohydrate? (insulin vs inulin)
Insulin is a protein (a polypeptide), so within each chain the amino acids are joined by peptide bonds. Do not confuse it with 'inulin', which is a polysaccharide (a polymer of fructose) joined by glycosidic bonds. NEET 2020 tested exactly this pair - insulin = peptide bond, inulin = glycosidic bond.
Does mature insulin contain the C-peptide?
No. The C-peptide belongs to proinsulin. It is cut out during maturation and is absent in the final mature insulin. So mature insulin = chain A + chain B + disulphide bridges, with no C-peptide.
⚠️ The NEET trap ✗ The A and B chains of insulin are held together by hydrogen bonds (or by a generic 'covalent bond', or mature insulin has three chains A, B and C). ✓ The A and B chains are held together by disulphide (S-S) bridges, which are a specific covalent linkage. Mature insulin has only two chains (A and B); the C-peptide is removed and is NOT in mature insulin. 🧠 Read the linkage word very carefully - NTA swaps one word to make a wrong option look right.
Real NEET questions
NEET 2016 Phase 1
The two polypeptides of human insulin are linked together by:
A · Hydrogen bonds
B · Phosphodiester bond
C · Covalent bond
D · Disulphide bridges ✓
Solution: Human insulin is made of two polypeptide chains (A and B) held together by covalent disulphide (S-S) bridges between cysteine residues. Hydrogen bonds only weakly stabilise folding, phosphodiester bonds link nucleotides in DNA/RNA, and 'covalent bond' is too generic - the specific answer is disulphide bridges.
NEET 2023 Phase 2
Select incorrect statement, regarding chemical structure of insulin.
A · C-peptide is not present in mature insulin molecule.
B · Polypeptide chains A and B are linked by disulphide bridges.
C · Mature insulin molecule consists of three polypeptide chains - A, B and C. ✓
D · Insulin is synthesized as prohormone which contains extra stretch of C-peptide.
Solution: Mature insulin has only TWO chains (A and B) linked by disulphide bridges, not three. The C-peptide is part of proinsulin and is removed during maturation, so it is absent in mature insulin. Hence the statement saying three chains (A, B and C) is the incorrect one.
NEET 2021
With regard to insulin choose correct options. (a) C-peptide is not present in mature insulin. (b) The insulin produced by rDNA technology has C-peptide. (c) The pro-insulin has C-peptide. (d) A-peptide and B-peptide of insulin are interconnected by disulphide bridges.
A · (a), (c) and (d) only ✓
B · (a) and (d) only
C · (b) and (d) only
D · (b) and (c) only
Solution: Proinsulin carries a C-peptide (c true); this C-peptide is removed during maturation so it is absent in mature insulin (a true); chains A and B are linked by disulphide bridges (d true). Statement (b) is false because rDNA insulin is assembled from separate A and B chains and has no C-peptide. So (a), (c) and (d) are correct.
Solved Biotechnology and Its Applications NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Why does NEET keep asking about disulphide bridges in insulin?
Because it is a clean, one-word trap: the correct linkage is disulphide bridges but 'hydrogen bonds' looks tempting. The same idea has appeared across NEET 2016, 2017, 2019, 2021 and 2023, so it is a guaranteed-marks concept if you remember 'A and B chains, disulphide bridges'.
How many disulphide bonds are in insulin? Do I need the number for NEET?
NCERT does not ask for a count, so for NEET you only need the idea that A and B chains are joined by disulphide (S-S) bridges. Do not memorise numbers that NCERT does not give; focus on 'disulphide bridges, not hydrogen bonds'.
Is insulin a peptide hormone or a steroid hormone?
Insulin is a peptide (protein) hormone, made of amino acids. This is why its chains carry peptide bonds and why the two chains can be joined by disulphide bridges between cysteine amino acids.
What is the difference between proinsulin and mature insulin?
Proinsulin is the single-chain immature form that still contains the extra C-peptide. Mature insulin is the final active form with only chains A and B (C-peptide removed) held by disulphide bridges. This is covered in the next concept, Proinsulin, C-peptide and how mature insulin forms.
